Development of a highly efficient ammonia synthesis process is desirable for achieving a sustainable society. Regarding conventional heterogeneous catalysts, Ru-supported catalyst exhibits higher turn-over frequency (TOF) than Fe-supported or Ni-supported catalysts. However, we found that Fe-supported and Ni-supported catalysts show higher TOF than Ru-supported catalyst in an electric field at the low temperature of 373 K. Density functional theory (DFT) calculations revealed that N-2 dissociation through the "associative mechanism" plays a key role in the electric field. The ammonia synthesis activity in the electric field is determined by the N2H formation energy at the metal-support interface.